Methods of thin layer preparation for radionuclide sources have been a subject of many publications and review articles (Van der Eijk, W., Oldenhof, W. and Zehner, W.  less than  less than Preparation of thin sources, a review greater than  greater than  Nud. Instr. and Meth. 112 (1973) 343; Lowenthal, G. C. and Wyllie, H. A. xe2x80x9cSpecial methods of source preparationxe2x80x9d. Nucl. Instr. and Meth. 112 (1973) 353). Amongst the applied techniques are: electrolytic or electrophoretic deposition, evaporation, sputtering or sublimation under vacuum, direct spraying or painting, precipitation and self transfer or drying of a liquid drop directly deposited on a substrate. Only the last method can be used for quantitative source preparation. All other methods require an independent determination of the deposition yield and hence, cannot be regarded as absolute source preparation methods.
Standardization of the activity concentration of radionuclide solutions by some important methods requires the preparation of solid samples with known amounts of the radioactive substance on a suitable substrate. The deposits should also be stable, well defined and rigidly bonded to the substrate. A uniform distribution of the source material over a large area on the supporting substrate minimizes self-absorption.
In most cases, drying of a liquid drop of a radioactive solution results in an agglomeration of crystals, the final size of which depends mainly on the time available for the crystals to grow The usual practice, as described by Van der Eijk and Zehner, in xe2x80x9cPreparation of thin sources for absolute beta-countingxe2x80x9d Radiochimica Acta 24, 205 (1977), was to dry the sources in a fume hood by the air draft.
The results were often unsatisfactory since large crystals were formed mainly at the boundary of the drop and at dust particle inclusions. In some cases the liquid withdrew to a much smaller area than the initial drop size, resulting in a very inhomogeneous distribution of the deposit.
The uniformity of the deposit was improved by stirring the drop with a dry nitrogen jet (Wyllie, H. A., Johnson, E. P. and Lowenthal, G. C.  less than  less than A procedure for stirring aliquots of radioactive solutions when making thin 4xcfx80 counting sources greater than  greater than . Int. J. Appl. Radiat. Isot. Vol 21,497 (1970)). However, due to the long drying time of typically 10 to 20 minutes, crystals could still grow to large sizes.
The object of the present invention is to provide a method and a device to prepare a thin and uniform deposit on a solid substrate by accelerated drying of a droplet of a solution containing dissolved crystalline material so as to obtain a solid residue as thin and uniform as possible, firmly bonded to the substrate without loss of dissolved matter.
In order to overcome the above mentioned problem, the present invention provides a method of thin layer preparation for a radionuclide source comprising the following steps:
deposition of a drop of a radionuclide dissolved in a solvent onto a on a substrate,
placing said drop in a confined space with a reduced pressure,
directing at least one flow of a hot gas onto the drop,
rotating the source relative to the hot gas jet creating turbulences inside the drop,
evaporating the solvent and obtaining a thin layer of dry radionuclide.
The method allows to prepare a thin and uniform deposit of a radionulcide or any other crystalline product on a solid substrate by accelerated drying of a drop of a solution containing dissolved radionulcide or crystalline material so as to obtain a solid residue as thin and uniform as possible, firmly bonded to the substrate without loss of dissolved matter.
According to a preferred embodiment the flow of a hot gas is lowered as the solvent is evaporated. It is also recommended to lower the temperature of the hot gas flow is lowered as the solvent is evaporated. Preferably, the temperature of the hot gas is varied from about 200xc2x0 C. to about 50xc2x0 C. For example, the temperature of the hot gas may be lowered to about 50xc2x0 C. as soon as about 70 to 90% of the solvent is evaporated. In general, the temperature of the gas flow is not critical.
In a preferred embodiment, at least four flows of hot gas are directed onto said drop and the hot gas flows are spaced symmetrically around said drop. At the beginning of the drying process, the gas flow can be moved from a stand-by position outside said drop towards just inside said drop boundary. The gas flow hit said drop preferably at four diametrically opposite positions so as to keep said drop safely confined.
In order to accelerate the method further, the gas flow is regulated in such a way that a depression is created in the drop and that said the depression does not reach the support substrate.
According to another aspect of the invention an Apparatus for thin layer preparation for a radionuclide source is proposed. The apparatus comprises:
block means with a moving mechanism,
heatable gas injector means movably fixed to said block means,
turntable means mounted on a shaft of a motor, said turntable means comprising a space for removably fixing a support means for said radionuclide source;
bell jar means connected to a vacuum pump for applying a reduced pressure to said bell jar means, said bell jar means covering said turntable means and said gas injector means.
said block means is supported by supporting means comprising power and gas supply means for said gas injector means;
said block means is mounted above said turntable in such a way that said gas injector means are directed towards said support means for said radionuclide source.
The motor may be e.g. a geared asynchronous motor using frequency control to vary rotation speed from 5 to 150 rpm.
According to another aspect of the invention, the gas injector means comprise four gas injectors placed symmetrically with an inclination in a distance above said turntable. Said distance between the gas injector means and the turntable as well as the inclination of the gas injectors are adjustable.